The invention relates to signal detectors and methods, for use for example in optical or electrical systems, and methods and apparatus for spectrum analysis.
In wavelength-division multiplexed (WDM) optical systems it is useful to detect channel power of channels of an optical signal as it propagates through a communications network. Channel power of individual channels of the WDM optical signal can be measured by de-multiplexing the WDM optical signal and then making a direct measurement but such a technique is expensive. To avoid this, in another approach [G. R. Hill, et al., xe2x80x9cA Transport Network Layer Based on Optical Network Elementsxe2x80x9d, Journal of Lightwave Technology, Vol.11, no. 5/6, pp.667-679, May/June 1993] each channel is modulated with one or more respective dither signal(s) resulting in each channel having a unique tone within its power spectral density, the remaining spectrum being that of the data carrying signal. The channel power for each channel is determined by identifying the respective dither signals and measuring the power of the respective dither signals. Detection of the channel power of individual channels becomes difficult when there are large variances in channel power between channels of the WDM optical signal. More specifically, the power spectrum associated with individual channels of a WDM optical signal may vary over a dynamic range up to 30 dB. Such a large dynamic range is due to, for example, channel add/drop throughout a communications network in which the WDM optical signal propagates with or without wavelength dependent attenuation along an optical fiber or wave-guide. In cases where the power spectral density of a WDM optical signal varies over a large dynamic range, the data spectrum density of more powerful channels may act as noise in the detection of less powerful channels. As such, optical systems using modulation techniques to detect channel power require very powerful DSPs (digital signal processors). These DSPs collect data for long periods of time up to (for example 100 s) for each channel to correctly identify channel power and this results in a long detection latency. The collection of data for such a long period of time requires extensive computations and large memories. The long detection latency effectively results in non-real-time detection of channel power, large memory requirements and a requirement for expensive DSPs. This solution is clearly impractical.
Provided are a spectrum analyzer, a signal detector and methods for spectrum analysis and for measuring power of one or more channels of an electrical or optical signal. Each channel may carry a unique modulation tone. The spectrum analyzer performs a DFT (discrete Fourier transform) on the signal. Only frequency bands of interest which contain a tone that need to be detected are processed. Higher layers of coherent integrations are performed on the frequency bands of interest which contain a modulation tone with a SNR (signal-to-noise ratio) which does not exceed a minimum threshold suitable for power measurement and thereby require finer resolution. The higher layer coherent integrations are performed by collecting additional data and performing a coherent integration. Further higher layers of coherent integrations are performed until all tones have been detected with a SNR exceeding the minimum threshold or a maximum detection latency has been reached. Processing only frequency bands of interest and performing higher layers of coherent integrations on only those bands of interest requiring a finer resolution provides a variable detection latency and efficient use of memory and computations thus allowing power measurements to be performed in real-time.
In accordance with a first broad aspect, the invention provides a method of performing a spectrum analysis. DFTs are performed upon a sequence of time domain measurements. The DFTs produce frequency domain samples associated with respective frequency bands. At least one higher layer of coherent integrations is then performed for at least one frequency sub-band of at least one of the respective frequency bands.
In some embodiments, the DFTs may be evaluated using a FFT (fast Fourier transform) algorithm. In such embodiments, of the respective frequency bands, only frequency bands of interest which carry a respective tone that requires detection may be monitored.
In some embodiments, frequency domain samples may be produced only for frequency bands of interest, of the respective frequency bands, which carry a respective tone that requires detection. A higher layer of coherent integrations may be performed within a layer j wherein jxe2x89xa72. Within layer j a number Rj of frequency domain samples within a previous layer jxe2x88x921 having identical center frequencies, fcjxe2x88x921,s, may be coherently integrated. The frequency domain samples within the previous layer jxe2x88x921 may be frequency domain samples of a frequency band or sub-band, s, of frequency bandwidth, xcex94fjxe2x88x921, within layer jxe2x88x921. The frequency domain samples within the previous layer jxe2x88x921 may be coherently integrated to produce frequency domain samples, within layer j, each having an associated frequency sub-band, t, of frequency bandwidth, xcex94fj=xcex94fjxe2x88x921/Rj. In some embodiments, at least one of the frequency domain samples within the previous layer jxe2x88x921 may be obtained from at least one additional sequence of time domain measurements. Furthermore, the at least one additional sequence of time domain measurements may be collected at a particular time interval. This time interval may allow the frequency domain samples within the previous layer jxe2x88x921 to be coherently integrated without having to apply a global phase shift to synchronize the frequency domain samples within said previous layer jxe2x88x921. In some embodiments, when being coherently integrated within the layer j, the frequency domain samples within the previous layer jxe2x88x921 may be synchronized using a twiddle factor, Wxcfx86gj(r)=exe2x88x92jxcfx86(r), wherein xcfx86gj(r) is a global phase shift. Furthermore, the global phase shift may satisfy xcfx86gj(r)=2xcfx80fcjxe2x88x921.sxcex94tr wherein xcex94tr may be a time interval between sampling of respective sequences, i and i+r, of time domain measurements associated with the frequency domain samples within the previous layer jxe2x88x921. The respective sequences, i and i+r, of time domain measurements may be sampled in a manner that the time interval, xcex94tr, may be an integral multiple of rN/fs wherein N may be a number of time domain measurements within each one of the sequences, i and i+r, of time domain measurements. fs may be a sampling frequency of the time domain measurements.
In some embodiments a local phase shift may be applied to the frequency domain samples within the previous layer jxe2x88x921. This may be done to allow the frequency domain samples within the previous layer jxe2x88x921 to be coherently integrated at center frequencies, fcj,t, different from the center frequencies, fcjxe2x88x921,s. In such embodiments, the center frequencies, fcj,t, may be center frequencies of the respective frequency sub-bands, t. As such the respective frequency sub-bands, t, which may be within said layer j, may be monitored. Furthermore, within the layer j, only frequency sub-bands within a subset of the respective frequency sub-bands, t, may be monitored.
The frequency domain samples within said previous layer jxe2x88x921, may be coherently integrated using a twiddle factor, Wxcfx86tj(t)=exe2x88x92jxcfx86tj(t) wherein xcfx86tj (t) may be a local phase shift. Incorporation of the twiddle factor may allow the respective frequency sub-bands, t, which may be within the layer, j, to be monitored. Furthermore, within the layer j, the local phase shift, xcfx86lj (t), may satisfy xcfx86lj(t)=2xcfx80txcex94fj.
In some embodiments, for each one of the frequency domain samples within the previous layer jxe2x88x921, coherent integrations may be performed for increasing values of j until the frequency bandwidth, xcex94fj, is small enough to detect a respective tone with suitable accuracy. Furthermore the coherent integrations may be stopped when a maximum detection latency has been reached.
The respective frequency domain samples produced from the DFTs may be saved in a memory as guard frames. Furthermore, the frequency domain samples within the layer j may also be saved in the memory as guard frames. In some cases any one or more of the time domain measurements may be corrupted. This may result in one or more of the guard frames being corrupted. Guard frames that are not corrupted may be used to re-calculate higher layers of coherent integrations to correct errors that may occur due to the corrupted time domain measurements.
The method may be used for detecting one or more channels of a signal. The signal may carry one or more frequencies and each one of the frequencies may have a unique modulation tone. Furthermore, only frequency bands of interest, of the respective frequency bands, which carry ones of the modulation tones that require detection may be monitored.
A DSP (digital signal processor) may implement the method to perform a spectrum analysis.
Another broad aspect of the invention provides a spectrum analyzer. The spectrum analyzer has input means used to collect sequences of time domain measurements of a signal. The spectrum analyzer has transform means used to perform DFTs upon the sequences of time domain measurements of the signal. The DFTs produce frequency domain samples associated with respective frequency bands. The spectrum analyzer also has integration means adapted to perform at least one higher layer of coherent integrations for at least one frequency sub-band of at least one of the respective frequency bands.
Another broad aspect of the invention provides a signal detector that is used to measure the power of one or more channels of a signal. The signal detector has a signal converter that converts a portion of the signal into a digital electrical signal. The signal detector also has a spectrum analyzer that performs DFTs upon at least one sequence of time domain power measurements of the digital electrical signal. The DFTs produce frequency domain samples each representing power of associated respective frequency bands. The spectrum analyzer also performs at least one higher layer of coherent integrations for at least one frequency sub-band of the respective frequency bands.
Yet another broad aspect of the invention provides an article of manufacture. The article of manufacture has a computer readable medium having computer readable program code means. The program code means is used to perform a spectrum analysis. The program code means in the article of manufacture has computer readable code means for performing DFTs upon at least one sequence of time domain power measurements. The DFTs produce frequency domain samples associated with respective frequency bands. The program code means has computer readable code means for monitoring frequency bands of interest, of the respective frequency bands, which carry tones that require detection. The program code means also has computer readable code means for performing one or more higher layers of coherent integrations. The higher layers of coherent integrations are performed for one or more of a plurality of frequency sub-bands of each one of the frequency bands of interest in which a signal has yet to be detected with a frequency bandwidth which is small enough for a sufficiently accurate power measurement.
The program code means may also have computer readable code means for determining the power associated with a respective one of the frequency domain samples associated with the respective frequency bands and the power associated with the frequency domain samples within a layer j of the higher layers of coherent integrations.